Methods of forming copper-containing sputtering targets

ABSTRACT

Described is a sputtering target assembly of high purity copper diffusion bonded to a precipitation hardened aluminum alloy backing plate via an intermediate layer of a CuCr alloy and in which the copper contains a micro alloy addition of at least one of Ag, Su, Te, In, Mg, B, Bi, Sb and/or P. Also disclosed is a method that includes preparation of a master alloy for addition to high purity copper and fabricating, heat treating and diffusion bonding processes to produce a sputtering target assembly with a stable fine-grained target microstructure.

BACKGROUND OF THE INVENTION

[0001] Interconnects for use in integrated circuits are generally madeof aluminum but aluminum is unsatisfactory in high speed semiconductordevices. The resistivity of aluminum as well as its electromigrationlimits and restrict its use in such applications. The present inventioncontemplates use of high purity copper as interconnects for use inintegrated circuits which have requirements of line width about 0.18 μmor less and are to be used in semiconductor devices at 1200 Mhz clockspeeds. The interconnect lines are created by sputtering high puritycopper from sputtering target assemblies.

[0002] To produce very small line widths, as described, we havedetermined that the sputtering target should be of high purity, i.e. atleast 99.999 wt. % purity, (referred to as 5N copper) and preferably99.9999 wt. % purity (referred to as 6N copper). Purity is important tomaintain the low resistivity of the copper line so that high speed goalscan be achieved. Moreover, purity may also influence electromigrationresistance.

[0003] We have also determined that it is also desirable that thesputtering target have a fine and substantially uniform grain size,preferably about 50 μm or less. The fine grain size aids in achieving auniform film thickness during deposition, allows the target to sputterfaster, and seems to result in fewer particle problems on the substrate,i.e. wafer. It is also important in accordance with the invention forthe target to be diffusion bonded to a light weight, high strengthbacking plate. As targets become larger, particularly those designed for300 mm diameter silicon wafers, the weight of the target becomes asignificant handling factor. Technically, it would be possible todecrease the thickness of the sputtering target surface because only apart of the target is consumed. However, in order to take advantage ofthis capability, it is necessary to bond a thinner target blank onto alightweight backing plate provided the thinness of the sputtering targetsurface does not allow the target assembly to warp during use. Warpingof the target can lead to inconsistent deposition as well as particlegeneration. Therefore, it is important that the backing plate be strongand stiff in addition to being lightweight. The present inventionachieves these objectives by providing a sputtering target assemblycomprising high purity copper target diffusion bonded to a precipitationhardened aluminum backing plate. By use or diffusion bonding to join thetarget to the backing plate it is possible to avoid the need forsoldering which is undesirable because temperatures required forsputtering often are sufficiently high to melt the solder bond of thetargets and, moreover, the heat generated has potential to continuegrain growth in the target after long periods of use.

SUMMARY OF THE INVENTION

[0004] In accordance with the invention there is provided a sputteringtarget assembly comprising a high purity copper target, a precipitationhardened aluminum alloy backing plate and an intermediate layer of CuCrdiffusion bonded to the target and backing plate. Desirably, thealuminum is in the fully hard T6 condition and the sputtering targetcomprises copper of a purity of at least about 99.999 wt. %. Thesputtering target also contains a micro-alloy grain stabilizerscomprising at least one of Ag, Sn, Te, In, Mg, B, Bi, Sb, and P. Thestabilizer is preferably present in an amount of about 0.3 ppm to 10pmm. The intermediate CuCr comprises copper and about 0.5 to 1.5 wt. %Cr, preferably about 1%. The high purity copper target has asubstantially uniform grain size of not more than about 50 μm.

[0005] In another embodiment of the invention there is provided a methodof making a sputtering target assembly which comprises providing highpurity copper target of at least about 99.999 wt. % purity. Preparing amaster alloy of copper and not more than about 10 ppm of at least one ofthe micro-alloy grain stabilizers described previously, preparing moltencombination of high purity copper and the master alloy and solidifyingthe molten combination to produce a cast billet; hot deforming the castbillet for a total of at least about 50% deformation on each of the axesand then rapidly quenching the deformed billet, preferably in water;frictionless forging the quenched billet at elevated temperature toabout 70% of the starting length of the billet and rapidly quenchingpreferably in water; cold rolling to a total of about 90% deformation;producing an aluminum alloy backing plate having a precladding surfaceof CuCr diffusion bonded thereto; diffusion bonding the high puritycopper target to the preclad CuCr surface; and precipitation hardeningthe aluminum alloy backing plate to the fully hard T6 condition.

[0006] Preferably the master alloy is prepared by combining a majoramount of high purity copper with a minor amount of at least one of themicro-alloy stabilizers Ag, Sn, Te, In, Mg, B, Bi, Sb, and P, meltingthe combination and casting to produce a master alloy. In the preferredembodiment the master alloy is formed by combining high purity copperwith at least one of the micro alloy stabilizers in a ratio of about1,000 to 1.

[0007] The backing plate having a precladding surface of CuCr diffusionbonded thereto is preferably produced by a process comprising embeddingan alloy of Cu and Cr in an aluminum or aluminum alloy envelope ande-beam welding the envelope closed in a vacuum environment; heattreating the enclosed envelope and forging to bring the CuCr intointimate contact with the aluminum alloy, and then heat treating todiffusion anneal and solutionize the hardening elements in the aluminumalloy, quenching and thereafter removing the aluminum alloy envelope toexpose the CuCr surface and precipitation harden the aluminum alloy to afully hard T6 condition.

[0008] Sputtering targets in accordance with the invention may be usedto produce an interconnect for use in an integrated circuit, having awidth 0.18 microns or less and comprising copper of at least about99.999 wt. % purity, preferably copper of at least about 99.9999 wt. %purity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a schematic diagram showing the orientation of thedeformation steps in the hot work breakdown of the cast copper billet;

[0010]FIG. 2 is a schematic flow diagram for diffusion bonding of a highpurity copper target;

[0011]FIG. 3 is a photomicrograph of a cross section of a coppercasting;

[0012]FIG. 4 is a diagram describing the sample locations on a Cucasting for an experimental matrix on deformation;

[0013]FIG. 5 is a schematic diagram describing the hot work breakdowndeformation schedule;

[0014]FIG. 6 is a photomicrograph of a sample having been subjected to a40% deformation;

[0015]FIG. 7 is a photomicrograph of a sample having been subjected to a30% deformation;

[0016]FIG. 8 is a photomicrograph of a sample subjected to a 40%deformation, as shown in FIG. 6, and 70% frictionless forging

[0017]FIG. 9 is a photomicrograph of a sample subjected to 30%deformation, as shown in FIG. 7 and a 70% frictionless forging;

[0018]FIGS. 10 and 11 are photomicrographs of a Cu target, CuCrintermediate layer and an Al backing plate; and

[0019]FIG. 12 is a photomicrograph of a Cu target microstructure at200×magnification, showing grain size of about 30 μm.

DETAILED DESCRIPTION

[0020] As previously discussed, high purity copper, for example of atleast 99.999 wt. % purity is especially useful for producing high puritycopper interconnects for use in integrated circuits. Control of grainsize and grain growth is very difficult for purity levels exceeding99.999 wt. % and containing unalloyed material. Also, copper isparticularly susceptible to anomalous grain growth effects, where a fewgrains scattered throughout the material grow considerably faster thanthe matrix. As the grains grow, they consume the smaller matrix grainsuntil the entire structure ends up with a very large grain size, oftenin excess of 250 μm. There is a critical temperature for the onset ofanomalous grain growth but unfortunately the critical temperature isoften above the minimum temperature needed for diffusion bonding.

[0021] It might be desirable to employ low diffusion bondingtemperatures so as not to de-stabilize the target grain structure, buthigh purity copper does not possess the inherent temperature stabilityto survive these conditions and if used, result in a weak bond strength,i.e. less than 4,000 psi. Also, the use of a low temperature bondingtechniques does nothing to stabilize the target microstructure duringheat generated during the sputtering process.

[0022] It has been found that through use of selected alloying elementsat ppm and sub-ppm levels, a high purity copper target microstructurecan be stabilized sufficiently to allow diffusion bonding and preventgrain growth under high power sputtering conditions that generate aconsiderable amount of heat. However, controllably alloying to the ppmand sub-ppm levels is difficult. The present invention solves thisproblem by use of a master alloy with precise control of the amounts ofmicro-alloying stabilizers. The amounts of alloying elements are atsufficiently low levels as to not interfere with the bulk resistivity ofthe copper. The stabilization of the structure provides some latitude intemperature parameters during diffusion bonding processes therebypermitting enough of a temperature window to provide a mechanicallystrong bond, e.g. greater than bout 4,000 psi, with a high stiffnessbacking plate.

[0023] The thermal mechanical processing used to make the targets has amajor affect on the final grain size and uniformity. The deformationprocess must be able to break up existing grains regardless of theirsize and orientation. Large deformation on all three axis of the castingbillet is required. An incorrect deformation process yieldinginsufficient deformation will result in some of the as cast grainstructure being retained in the final structure as larger grains thatare partially or fully re-crystallized. Such large-grained areas caninterfere with the sputtering performance of the target. Correctdeformation procedures will result in a fine grain and substantiallyuniform structure in the target.

[0024] To produce a sputtering target assembly in accordance with theinvention a high purity copper target is diffusion bonded to aprecipitation hardened aluminum alloy backing plate by use of anintermediate CuCr interlayer between the high purity copper target andthe aluminum backing plate. This provides a final bonded target assemblywith several unique properties. Firstly, the grain size of the highpurity copper target remains small, e.g. less than about 50 μm andsubstantially uniform, i.e. with no anomalous grain growth. Secondly,the strength of the precipitation hardened backing plate is in the fullyhard T6 condition, which is an important requirement for large, thinplanar sputtering targets. This technique could be used withprecipitation hardenable Al alloys, such as the 2000, 6000 or 7000series alloys. It is also possible to achieve diffusion to anon-precipitation hardenable alloy, such as the 5000 series alloy, butthe backing plate will not achieve a fully hard condition, with alloy6061 being the most popular grade.

[0025] In addition to the CuCr alloy described, there are a number ofcommercial Cu alloys that could be used in this process.

[0026] These alloys are included in the following: TABLE 1 *UNS #Copper/Copper Alloy Types C10100 Oxygen-free coppers C10200 C10300C10400 Oxygen-free silver coppers C10500 C50700 C10800 Oxygen-free lowphosphorus copper C11000 Electrolytic tough pitch coppers C11300 Silverbearing, tough pitch coppers C11400 C11500 C11600 C12500 Fine refined,tough pitch coppers with silver C12700 C12800 C12900 C13000 C14300Cadmium coppers C14310 C16200 C14500 Free machining coppers C18700C14700 Sulfur copper C15000 Zirconium copper C17000 Beryllium coppersC17200 C17300 C17500 C17600 C18200 Chromium coppers C18400 C18500 C19200High strength modified coppers C19400 C19500 C60600 Aluminum bronzeC65100 Low silicon bronze # designations are simply expansions of theformer designations. For example, Copper Alloy No. 377 (forging brass)in the original three-digit system became C37700 in the UNS System. TheUNS is managed jointly by the American Society for Testing and Materialsand the Society of Automotive Engineers. Because these old numbers areembedded in the new UNS numbers, no confusion need result.

[0027] In one example of the method of producing a high puritysputtering target assembly in accordance with the invention, a highpurity copper target of at least about 99.999 wt. % purity, preferablyat least about 99.9999 wt. % purity, is provided. A high purity Cutarget can be made by a casting process that includes consolidation ofultra-high purity, electro-refined copper cathode plate. The cathodeplate is melted in order to allow residual chemicals from the refiningprocess to be burned off. Surface moisture from the environment is alsodriven off during melting, and the melt is held at a high enoughtemperature to allow a majority of the gases and volatilizable materialentrapped in the metal, such as oxygen and sulfur, to be driven from themetal. The metal is then allowed to solidify in the crucible.

[0028] An important step in the copper casting process is to prepare amaster alloy. Although there many alloying elements have the ability toraise the recrystallization temperature of pure copper, the requiredalloying level has been thought to be typically higher than 1000 ppm.However, in accordance with the present invention it has been found thatsignificant increases in the recrystallization temperature can beachieved with lesser amounts of selected alloying elements, i.e. lessthan about 10 ppm, or even sub-ppm level additions, but the potentialalloying elements must be chosen so as not to interfere with theresistivity of the metal. Alloying elements that meet this criteriainclude, but are not limited to, Ag, Sn, Te, In, Mg, B, Bi, Sb, and P.As an example, silver may be added in the range of about 0.3 ppm to 10ppm to act as a stabilizer for the grain structure during diffusionbonding, while still allowing the Cu target to meet the minimum 99.999wt. % purity grade specifications. A preferred amount of silver is about2.5 ppm in this context. However, to make the master alloy with silver,or any of the other micro-alloy elements, care must be taken to avoidsimply adding the pure form of the element to copper since this willresult in a loss of significant amounts of the element due to the factthat their vapor pressure may exceed 1 atmosphere at the temperaturestypically used for melting copper. Because the vapor pressure is high,the alloying addition, e.g. silver, would evaporate, making it difficultto control the ppm or sub-ppm level of the alloying additions. Toprecisely control the alloying level, a master alloy, e.g. with at least100 ppm silver, is used. To make this master alloy a ratio of copper toalloying addition of about 1,000 to 1 is desirable. For example, 2000grams of high purity copper cathode plate is melted with 2 grams ofsilver. The metal may be melted via induction heating in an openatmosphere. Once the metal becomes molten, the melt is cast intoone-inch diameter billets. A sample of the billet is cut and analyzed bythe GDMS (Glow discharge mass spectrometer) analysis method. The smalldiameter billet of the master alloy can then be easily cut intoconvenient sizes for adding as alloying addition.

[0029] After the master alloy has been made, it can be combined inappropriate amounts with pure high purity copper to be melted to form acast billet. This operation is advantageously conducted in a vacuumfurnace in order to minimize the possibility of oxidation. Thepre-consolidated cathode plate is weighed to provide an amount whichwill produce the desired billet size, then the master alloy is cut andweighed to the desired alloying level, e.g. 2.5 ppm silver. Thepre-consolidated copper and master alloy is then placed in a meltingcrucible in a vacuum chamber. In the preferred embodiment the materialwill be heated until it reaches about 1200° C. to melt at whichtemperature it is held for about ten minutes and then cooled to about1038° C. The cycle is repeated two additional times to drive off anyresidual gas in the metal under vacuum environment. The metal is thenheated to the casting temperature and poured into a graphite mold andcooled. The top 3 inches and bottom 2 inches may be cut from the castbillet to remove any shrinkage or solidification voids at the top of thebillet and cold laps at the bottom of the billet. The billet is thenready for deformation.

[0030] The billet is deformed to remove the as-cast grain structure.This deformation step is referred to as the “hot work breakdown” of thebillet.

[0031] Again in the preferred embodiment the billet is processed asfollows: The billet is first heated to about 343° C. and then deformedfor a total of 50% deformation on each of the 3 axes. FIG. 1 shows theorientation of the deformation steps and the hot work deformationschedule. The billet is water quenched after the sixth hot workbreakdown step.

[0032] The next step is frictionless forging. The billet is heated toabout 343° C. and forged in a friction free condition to 70% of thestarting height using the same axis as the first hot work breakdowndeformation step. The sample is then water quenched. Next, the materialis cold rolled to give a total of 90% deformation (frictionlessforge+cold roll). The material is then ready for diffusion bonding.

[0033] The process for diffusion bonding process is shown generally inFIG. 2.

[0034] The first step in the diffusion bonding process involves thesimultaneous diffusion bonding of the CuCr to the precipitationhardenable aluminum alloy and the solutionizing of the aluminum alloy. Athin alloy plate of copper with about 0.5 to 1.5 wt. % chromium is firstembedded inside an envelope of aluminum alloy as shown in FIG. 2. Thisassembly is then e-beam welded shut in a vacuum environment. The sealedassembly can be removed from the vacuum environment and handled freelywithout fear of contamination or oxidation. The assembly is heated to338° C., and the temperature is allowed to equilibrate for 30 minutes.

[0035] After 30 minutes at 338° C., the assembly is forged at 300° C. tobring the CuCr and the aluminum alloy 6061 into intimate contact. Theassembly is then quickly returned to the furnace for a 1-hour diffusionanneal at 338° C. After 1 hour at 338° C., the furnace temperature isthen slowly ramped to 529° C. and held at this temperature for 90minutes to put all the hardening elements in the 6061 alloy intosolution. This solutionizing step is followed immediately by a rapidquench, e.g. in water. Quenching is critical in order to retain thehardening elements in solution for subsequent precipitation hardening.Quenching differentiates this process from HIP or vacuum hot pressprocesses, since quenching would not be possible in the latter twocases. After quenching, the cover of the assembly is machined off toexpose the CuCr surface.

[0036] The pre-clad backing plate is now ready for diffusion bonding tothe Cu sputtering target blank. A cleaned Cu blank is stacked on thecleaned CuCr surface, and the stack is placed into a vacuum hot presswhere it is hot pressed under 8,500 psi of pressure at about 250° C. and300° C. for 2 hours, and furnace cooled to room temperature. Severalcritical transformations in the target fabrication process occur duringthis step. First the Cu blank is recrystallized to form a fine grained,uniform and stable microstructure. Second, the diffusion bond is formedbetween the Cu blank and the CuCr interlayer. Finally, the 6061 aluminumalloy backing plate is precipitation hardened to the fully hard T6condition.

[0037] The thermal mechanical processing used will have an affect on thefinal grain size and uniformity. Incorrect deformation procedure willresult in some of the as cast grain structure being retained in thefinal structure as larger grains that are partially or fullyre-crystallized. To elucidate the difference in final metallurgy between30% and 40% deformation during the hot work breakdown steps anexperimental matrix was established. Samples were chosen to ensure thata direct comparison would be possible between the two starting samples.

[0038] A cast billet is symmetrical about a vertical line running downthe center of the billet, see FIG. 3. Two samples cut from the top ofthe ingot and hot work on each axis by different amount will result indifferent grain sizes. As shown in FIG. 4, Sample A and B are from aregion of the casting were equiaxed largest grains are present. Sample Ais hot worked to get 40% deformation on each axis; Sample B is hotworked by 30% on each axis. The hot work breakdown schedule is describedin FIG. 5.

[0039] Samples, polished and etched, reveal that the microstructure ofthe 40% deformed sample was small grained and uniform. The 30% deformedsample has a non uniform grain structure, see FIGS. 6 and 7, and FIGS. 8and 9.

[0040] As can be seen, the hot work samples that are rolled or forged inone direction will have a grain structure and size that is dependent onthe as hot-worked grains. Microstructures of Samples A and Bfrictionless forged 70% in one direction as shown in FIGS. 6 and 7 arein the polished and etched state. The amount of deformation done duringthe hot work breakdown has an effect on final grain size, as indicatedin the photomicrographs of FIGS. 8 and 9. Clearly, the grain size ofFIG. 8 is significantly finer. FIGS. 10 and 11 show the assembly of a Cutarget Al backing plate and intermediate CuCr interlayer. FIG. 12 showsthe fine grain size of the high purity Cu target.

[0041] Recommended alloying elements include Ag, Sn, Te, In, Mg, B, Bi,Sb, and P. Silver is preferred to minimize the bulk resistivity of thematerial while maximizing the grain stabilization effect. Concentrationrange of the alloying elements are to a certain degree defined by grade.Recommended minimum and maximum concentrations are listed in Table 1.Below the minimum alloying levels, the grain structure fails tostabilize and anomalous grain growth can occur if the target is exposedto diffusion bonding temperatures. Above the maximum alloying levels,the material will no longer meet grade requirements for purity, if themicro-alloying element is considered in the total metallic impurities.TABLE 2 Recommended Alloying Ranges in ppm by Weight Minimum AlloyingMaximum Alloying Grade Level Level 5N (99.999% pure) 0.3 ppm 10 ppm 5N5(99.9995% pure) 0.3 ppm  5ppm 6N (99.9999% pure) 0.3 ppm  1 ppm

[0042] The optimum ranges for deformation are generally defined by theamount of deformation required to achieve a uniform structure. For thehot work breakdown step of the Cu deformation process, the total strainafter forging should advantageously be in the range of 2.14 and 5.49,corresponding to a 30% and a 60% forged height reduction per billetface. Below the minimum strain value, the structure may beinsufficiently worked and the uniform grain structure may not be able tobe achieved. There is risk that anomalous grains may result in thematerial due to insufficient deformation of isolated large grains thathappen to be unfavorably oriented for slip during forging. Above themaximum strain value, buckling may occur. Material that is folded duringbuckling will produce a highly non-uniform structure and will form largevoids within the material. Optimum deformation for the process employs atotal strain of about 4.15, with an average of about 50% forgedreduction per face. TABLE 3 Optimum Compositions For Alloying With Ag:Optimum Alloying Grade Level 5N (99.9995% pure)   5 +/− 0.5 ppm 5N5(99.9995% pure) 2.5 +/− 0.25 ppm 6N (99.9999% pure) 0.5 +/− 0.1 ppm

[0043] Total cold deformation after hot work breakdown, whether done byforging, rolling, an alternative deformation method, or some combinationof various cold deformation techniques, should result in a total heightreduction in the range of about 84% and 93%, with the optimum beingabout 90%. Below this range, sufficient grain refining may not occur andthe final product grain size may be too large. Above this range, theexcess strain causes the recrystallization temperature and anomalousgrain growth onset temperature to drop below the diffusion bondtemperature.

[0044] It is apparent from the foregoing that various changes andmodification may be made without departing from the invention.Accordingly, the scope of the invention should be measured only by theappended claims wherein what is claimed is.

1. A sputtering target assembly comprising a high purity copper target, a precipitation hardened aluminum alloy backing plate and an intermediate layer of CuCr diffusion bonded to the target and backing plate.
 2. A sputtering target assembly according to claim 1 wherein the backing plate is in the fully hard T6 condition.
 3. A sputtering target assembly according to claim 1 wherein the high purity copper target comprises copper of a purity of at least about 99.999 wt. %.
 4. A sputtering target assembly according to claim 1 wherein the high purity copper target comprises copper of a purity of at least about 99.995 wt. %.
 5. A sputtering target assembly according to claim 3 wherein the sputtering target contains a micro-alloy grain stabilizer comprising at least one of Ag, Sn, Te, In, Mg, B, Bi, Sb, and P.
 6. A sputtering target assembly according to claim 4 wherein said micro-alloy grain stabilizer is present in an amount of about 0.3 ppm to 10 ppm.
 7. A sputtering target assembly according to claim 5 wherein said micro-alloy grain stabilizer comprises Ag.
 8. A sputtering target assembly according to claim 1 wherein said intermediate layer comprises copper and about 0.5 to 1.5 wt. % Cr.
 9. A sputtering target assembly according to claim 1 wherein the copper target has substantially uniform grain size comprising not more than about 50 μm.
 10. A method of making a sputtering target assembly comprising: a) providing high purity copper target of at least about 99.999 wt. % purity; b) preparing a master alloy of copper and not more than about 10 ppm of at least one of Ag, Sn, Te, In, Mg, B, Bi, Sb, and P; c) preparing a molten combination of copper and master alloy and solidify the molten combination to produce a cast billet; d) hot deforming the cast billet for a total of at least about 50% deformation on each axis and then rapidly quenching the deformed billet, preferably in water; e) frictionless forging the quenched billet at elevated temperature to about 70% of the starting length of the billet and rapidly quenching, preferably in water; f) cold rolling to a total of about 90% deformation; g) providing an aluminum alloy backing plate having a precladding surface of CuCr diffusion bonded thereto; h) diffusion bonding said high purity copper target to the preclad CuCr surface; and i) precipitation hardening the aluminum alloy backing plate to the fully hard T6 condition.
 11. A method according to claim 9 wherein said master alloy is prepared by combining a major amount of high purity copper with a minor amount of at least one of Ag, Sn, Ti, In, Mg, B, Bi, Sb and P, melting the combination and casting to produce a master alloy.
 12. A method according to claim 10 wherein high purity copper is combined with at least one of Ag, Sn, Ti, In, Mg, B, Bi, Sb and/or P in a ratio of about 1000 to
 1. 13. A method according to claim 9 wherein the aluminum alloy backing plate having a precladding surface of CuCr diffusion bonded thereto is used which is produced by a process comprising embedding an alloy of Cu and Cr in an aluminum or aluminum alloy envelope and e-beam welding the envelope closed in a vacuum environment; heat treating the enclosed envelope and forging to bring the CuCr into intimate contact with the aluminum alloy to be used as a backing plate, quenching removing the aluminum alloy envelope to expose the CuCr surface and precipitation harden the aluminum alloy to full hard T6 condition.
 14. An interconnect for use in an integrated circuit, having a width of about 0.18 μm or less comprising at least 99.999% copper and a micro-alloy stabilizer of at lest one of Ag, Sn, Tr, In, Mg, B, Bi, Sb and/or P.
 15. An interconnect for use in an integrated circuit according to claim 13 comprising at least 99.9999% purity.
 16. An interconnect according to claim 14 wherein the amount of micro-alloy stabilizer is about 0.3 to 10 ppm.
 17. An interconnect according to claim 15 wherein the amount of micro-alloy stabilizer is about 0.3 to 10 ppm. 